The invention concerns a method and an apparatus to image an organ of the human or animal body, particularly for imaging a beating heart via a rotating acquisition device.
In modem medicine, manifold minimally-invasive methods to image an organ are known, such as x-ray methods. It is the goal of these methods to obtain a comprehensive knowledge of the respective organ and its state primarily without opening the body. In a known application, for example, a C-arm of an x-ray apparatus with an x-ray tube and an x-ray detector is rotated around a patent, normally around the patient longitudinal axis, with a constant rotation speed or angular velocity w over an angle of, for example, 300 degrees. Instead of an only platform view through a catheter with camera, etc., with such imaging methods and a corresponding apparatus, a number of individual exposures of the appertaining organs can be acquired from various spatial directions, on whose basis three-dimensional images or other representations (such as, in particular, arbitrary cross-sections) can ultimately be produced. With these methods, for example, the heart muscle and the coronary artery can be examined without a catheter.
However, for a 3D reconstruction, only those images can be used that show the organ in a respectively constant state. In a preferred and most important application case, the exposure of the heart, the filling phase or diastole, as a relative rest phase of the heart, is selected as a representation state. In a living human and in a rest position given relaxation, this rest phase lasts less than 200 milliseconds. During the acquisition, however, only a few projections for the 3D reconstruction and modeling can be collected by the moving acquisition device in a time window Δt in the rest phase of the heart cited above.
The respective data collected outside of the acquisition time window Δt cannot be used for imaging due to the heart movement. These data losses lead to large gaps in the space of the projection angle α, and therewith to an incomplete representation basis of the entire organ. At best, interpolations with relatively uncertain assumptions can be effected within these representation gaps. A similar problem also occurs with other organs whose shape and/or positions change over time. However, for reasons of keeping low a total radiation exposure with an x-ray tube continuously located in operation, the acquisition device may not exhibit an arbitrarily slow rotation speed in order to acquire optimally many exposures from different spatial directions at advantageous acquisition moments. For the same reasons, a measurement can also not be repeated arbitrarily often.
For this reason, U.S. Pat. No. 6,324,254 discloses the acquisition of a rhythmically moving vessel to in fact move the C-arm with an optimally slow rotation speed, preferably >2° per second to thereby implement individual image acquisitions triggered by the vessel movement or an organ movement causing the movement of the vessel. This means radiation is only respectively emitted by the x-ray tube and an exposure made at specific times while the C-arm moves around the patent with an optimally slow, constant speed.
The constant rotation speed is hereby preferably established before the measurement dependent on the frequency of the rhythmic vessel or organ movement in order to ensure that a minimum number of exposures can be generated during a measurement. However, this procedure has the disadvantage that the measurement lasts a relatively long time. In this time, the patent must lie absolutely still. Even minimal position changes can lead to impaired measurement.
A similar method is disclosed in U.S. Pat. No. 6,370,217 for measuring a periodically moving subject with a computer tomograph. Given a measurement with such a computer tomography system, the x-ray tube rotates multiple times around the patient with a very high speed, for example, with a speed in the range of approximately 1 s/rotation. In order to thereby always acquire the cyclically changing subject in the same state, here as well when the x-ray tube momentarily emits x-ray radiation, the imaging is controlled to be triggered by the movement cycle in order to thus make individual exposures.
Likewise, a constant rotation speed is hereby preferably determined before the measurement dependent on the frequency of the rhythmic vessel or organ movement. The rotation speed is thereby adjusted such that exposures are optimally generated from all spatial directions during a measurement This method Is, however, not usable for a measurement with a C-arm or similar acquisition device, since in such a relatively simple acquisition device, the x-ray tube and the detector can not be moved arbitrarily fast and often around the patient (this is different than in a significantly more elaborately constructed computer tomography apparatus). Typically, only a rotation angle of maximally 300° is available.